1. Field of the Invention
The present invention relates to an apparatus for drilling a printed circuit board using a drill.
2. Description of the Related Art
In the drilling of printed circuit boards to form through-holes, printed circuit boards 1 are generally stacked between an entry board 2 and a back up board 3 as shown in FIG. 5. A drill 4 rotated at a predetermined speed is disposed above this stack and fed until the bit thereof reaches the back up board 3.
With this drilling method, if three 1.6 mm thick printed circuit boards 1 are stacked on top of each other, if the entry and back-up boards 2, 3 placed above and below this stack have thicknesses of 1 mm and 3 mm, respectively, and if the drilled hole is to drill the back up board 3 by 1 mm, the depth Lh of the hole is 6.8 mm.
If the diameter of the hole is 1.2 mm, the ratio (Lh/D) of the hole depth Lh to the hole diameter D is 5.8.
When the aspect ratio of the hole depth Lh to the hole diameter D is relatively small (6 or less) as in the above example, it is possible to drill a good-quality, accurately-located hole by feeding the drill 4 from a drill resting position or drill start position A to a hole bottom position or drill end position B at a predetermined drilling feed rate then feeding it back from the hole bottom position B to the drill start position A at a predetermined rapid feed rate.
On the other hand, if the hole diameter D is 1 mm or less, e.g., 0.8 mm, the ratio of the hole depth Lh to the hole diameter D is 8.5.
If a hole of such a large ratio of Lh to D is drilled by feeding the drill 4 from the drill start position A to the hole bottom position B is one stage process, hole is often choked with debris. This increases the surface roughness of the inner wall of the drilled hole, and may also cause smears. Further, the thrust load on the drill 4 increases during drilling, rises drill temperature. This in turn increases the wear of the drill bit.
Accordingly, a proposal has been made to make a hole by drilling it in several stages, as shown in FIG. 6.
For example, the distance between the drill start position A and the hole bottom position B is divided into three stages with intermediate positions M.sub.1 and M.sub.2 set partway down the hole. The drill is fed from the drill start position A to the intermediate position M.sub.1 at a drilling feed rate V.sub.F, then back to the drill start position A at a rapid feed rate V.sub.R. Subsequently, the drill is fed from the drill start position A at the rapid feed rate V.sub.R to the intermediate position M.sub.1, at which the drill feed rate is switched over to the drilling feed rate V.sub.F, is fed further down at the drilling feed rate V.sub.F to the intermediate position M.sub.2, and is then fed back at the rapid feed rate V.sub.R to the drill start position A from the intermediate position M.sub.2. Thereafter, the drill is fed from the drill start position A at the rapid feed rate V.sub.R to the intermediate position M.sub.2, at which the drill feed rate is switched over to the drilling feed rate V.sub.F, is fed further at the drilling feed rate V.sub.F to the hole bottom position B, and is fed back at the rapid feed rate V.sub.R to the drill start position A.
In the above-described method, since the drill is fed retracted between the drill start position A and the intermediate positions M.sub.1 and M.sub.2 to make the hole, chips filling the drill flute are shaken off and discharged while the drill is retracting to the drill start position A after coming out of the entry board. Therefore, the problems caused by chip clogging are eliminated, and the drilling of a good quality hole is enabled. In addition, drill bit wear can be reduced.
If the diameter of a hole to be drilled is even smaller and so the diameter of the drill to be used is even smaller, the rigidity of the drill is reduced. In such a case, the above-described method has a disadvantage in that, when the drill bit makes contact with the entry board or the printed circuit board, it may not enter the desired part of the entry board or printed circuit board; the top end of the drill bit may slip sideways from a desired location where the board is to be drilled. This results in a reduction in hole positioning accuracy, or breakage of the drill.
Furthermore, since the drill is fed and retracted between the drill start position and the intermediate positions to drill, the time required for drilling is extended, thereby reducing the output. More specifically, a type of printed circuit board may require several thousands or several tens of thousands of holes. Therefore, if the time required to drill one hole increases by 0.1 second, it means that it will take between several minutes to several hours longer to complete the drilling of the printed circuit board.